1. Field of the Invention
The present invention relates to display devices and more particularly, to programmable displays for use on vehicles and about stations.
2. Description of the Related Art
Existing mass transit vehicles, such as buses and trains, carry destination and other signs for the purpose of conveying information to passengers using the mass transit vehicles. Destination signs inform passengers outside of the vehicle of the route information (route number and description). These signs may transmit information through a variety of display mechanisms. A sign may use light emitting diodes (LEDs), flip dot technology, or liquid crystal displays (LCDs), for example, in order to present alphanumeric information to the passengers. The destination signs normally are placed at the front of a mass transit vehicle over the windshield. The signs are mounted to the existing vehicle structure with various mounting brackets.
Typically, as shown in FIG. 1A, conventional destination signs utilize a chassis or frame assembly 100 constructed of steel or aluminum sheet metal to house the signs and other components. The sheet metal housing 10 is generally constructed in a rectangular box form with an opening, usually covered by transparent material 12, to enclose the entire housing 10 while still allowing passengers to easily read the alphanumeric characters generated by the sign. Various components are mounted to the housing 10 in order to prevent the components from shifting and breaking. In one embodiment, one such component is an LED board 16 with LEDs 14 coupled thereto that are controlled by a controller 18, which also is secured in the sheet metal housing 10 as shown in FIG. 1B. Other embodiments include LCDs and flip dot assemblies and the electronics associated therewith. Frame assembly 100 is mounted so that the LED board 16 is vertically upright, to permit passengers to view the characters formed by LEDs 14. Also enclosed in the housing 10 are the power supply and control board, as well as any other essential components needed for the frame assembly 100.
FIG. 1B illustrates an exploded view of the frame assembly 100 depicted in FIG. 1A. The exploded view shows the interior of the sheet metal housing 10 and some of the components typically secured within the housing 10. A louver 11, which minimizes glare to the LEDs, and the LED board 16 are secured to the housing 10 by fasteners 13, such as bolts or screws, along the lower edge of the housing 10, louver 11, and LED board 16 (a louver is optional for LCDs and flip dots). Such fasteners 13 thus compress the LED board 16 to the housing 10, but penetrate the LED board 16 to do so. By directly fastening the LED board 16 via the holes and fasteners 13 to the housing 10, a large amount of stress concentrates at the holes on the LED board 16. Any bending or torsion placed on the frame assembly 100 is translated to stress in the material adjacent to the holes of the LED board 16, often causing the LED board 16 to crack or cause electronics, including the display mechanisms, to deteriorate over time. Because mass transit vehicles are prone to vibration inducing or dynamic environments, failure rates of the conventional signs is high.
Furthermore, the LED board 16 also includes heat-emitting elements (not shown) that are covered by thermally conductive foam. The fasteners 13, penetrating LED board 16, press the foam to the LED board 16 and to the heat-emitting elements (not shown). The heat emitting elements generally include electronics, such as processors, power devices, etc. It is typical to use numerous fasteners 13 penetrating the LED board 16 to accomplish this. The fasteners 13 also are applied directly to LED board 16, thereby producing a concentrated stress around the area of the fasteners 13. If the fasteners 13 are over-tightened, then the LED board 16 cracks. Also inside the sheet metal housing 10 is a cable assembly 22 that transmits information and power to the LED boards 16 from the controller 18. Several cover assemblies 20 are secured to the LED board 16 and housing 10 to prevent movement or shifting of the components within the housing 10.
To further demonstrate the stress placed along the lower edge of the LED board 16, deflection measurements were taken at a plurality of locations along the lower edge of the LED board 16. Measurements of the fasteners 22 were taken and averaged to yield an average height of 0.320. These fasteners 22 are inserted at points 1, 3, 5, 7, 9, 11, and 13 in the lower edge of the LED board 16. Deflection of the LED board 16 at points (2, 4, 6, 8, 10, and 12) midway between the fasteners 22 is measured against the average height of the fasteners 22 height to determine if significant distortion occurs along the length of the LED board 16.
TABLE 1Exemplary Deflection MeasurementsAveragePositionTest 1Test 2Test 3Test 4Test 5Test 6Test 7Test 8Test 9AverageDistortion10.3190.3210.3160.3210.3200.3100.3120.3120.3120.3160.00420.3260.3270.3160.3220.3230.3140.3270.3170.3170.3210.00130.3240.3250.3160.3200.3230.3140.3260.3150.3140.3200.00040.3300.3300.3190.3240.3260.3170.3280.3190.3190.3240.00450.3240.3290.3160.3190.3220.3160.3250.3190.3150.3210.00160.3240.3300.3160.3170.3210.3150.3240.3180.3150.3200.00070.3230.3270.3160.3190.3220.3150.3250.3200.3170.3200.00080.3230.3250.3170.3210.3210.3170.3250.3200.3170.3210.00190.3230.3290.3170.3210.3210.3150.3250.3200.3170.3210.001100.3300.3300.3230.3250.3260.3180.3290.3230.3210.3250.005110.3240.3000.3190.3270.3250.3170.3250.3230.3180.3200.000120.3280.3300.3240.3290.3260.3180.3280.3250.3240.3260.006130.3230.3260.3200.3250.3210.3170.3210.3210.3170.3210.001As shown in TABLE 1, the deflection of the LED board 16 is substantial at the midway points across the lower edge. The distortion reaches as much as 0.006 at the plurality of measurement points along the lower edge of the LED board 16. As shown by the test results in TABLE 1, the considerable amount of distortion of the LED board 16 decreases heat transfer from the heat-emitting elements to the heat sink. Due to the flexibility of the LED board 16, a variation of up to (i) 10% in the deflection of the thermally conductive foam and (ii) 22% in the pressure applied to the heat-emitting elements (not shown) and thermally conductive foam severely decreases the effective thermal conductivity for the display.
The frame assemblies 100, as shown in FIGS. 1A and 1B, are stand-alone units, which are shipped and mounted to the mass transit vehicle in a single piece (i.e., as a single display unit). While multiple LED boards 16 (or LCDs or flip dot assemblies) may be utilized to form a complete display sign, the configuration of the conventional display signs is an integration of the LED boards 16 with the frame assembly 100 to form a sign. For the purposes of this discussion, the signs having the frame assembly 100 structurally coupled to each LED board 16 to form a housing of the display is considered to be non-modular. These frame assemblies 100 with the integrated LED boards 16 are usually four to six feet in length and can weigh from 30 to 50 pounds. Due to the cumbersome size and weight of the frame assemblies 100, shipping costs are high and at least two people are needed to maneuver and install the sign.
To install the sign, each of the upper and lower corners of the frame assembly 100 are mounted to the mass transit vehicle to secure the frame assembly 100 from shifting during the transport of passengers. If any portion of the sign, including the LED boards 16 and frame assembly 100, malfunctions, the repair process is very tedious and time-consuming even though the malfunction itself may be trivial. In addition, the frame assemblies 100 typically installed in mass transit vehicles may not adequately withstand many of the stresses associated with a moving vehicle. When these frame assemblies 100 are mounted to the mass transit vehicle, a frame assembly receives the stress and torque from the movement of the mass transit vehicle. When a mass transit vehicle turns, the side walls of the vehicle, ordinarily parallel to each other and perpendicular to the ceiling and floor, may shift angularly relative to the floor and ceiling of the mass transit vehicle so as to be non-perpendicular. In other words, the frame assembly 100 is constrained at the four corners forming a rectangle and stressed toward forming a non-rectangular parallelogram. As understood in the art, an over-constrained sheet metal housing is stressed by the shifting and may be pulled apart or distorted under such forces.
Therefore, there is a need for an easily installed and easily repaired destination sign capable of withstanding the stresses exerted by a mass transit vehicle. There is also a need for a destination sign capable of dissipating heat without causing significant stress to the LED board.